Don't get your hopes up. Although nothing has been solidly confirmed by the studio, I'd be willing to bet money that Emily is playing Sharon. Sharon is pretty much the only character that makes sense.

I think that maybe the reason Marvel hasn't confirmed anything yet is that, at least for the first part of the film, her character will be known only as Agent 13(with the reveal of her being Sharon and related to Peggy coming as a surprise for Cap later on in the film). Now, anyone familiar with the comics will know who Agent 13 really is, but Cap won't, and neither will many of the fans who are only familiar with the movies. My theory is that the film will begin with Agent 13 being captured by The Winter Soldier during a mission and then move to Cap and his team being sent in to rescue her. If that is the case, I could definitely see SHIELD wanting to keep her real identity a secret from Cap.

You have to remember though, movie studios don't really consider making sure we know every little detail about who certain actors are playing to be a huge priority. They have their own timeline when it comes to releasing info to the public and unfortunately, such timelines rarely make much sense to us. I suspect that in an effort to get the jump on any fan photos from the set, Marvel will let us know much more shortly before filming starts. Just be glad they tell us anything at all. I still remember what it was like before the internet took off... inside info about movies was basically non-existent back then.